Method and apparatus for cleaning a vacuum line in a CVD system

ABSTRACT

An apparatus for preventing particulate matter and residue build-up within a vacuum exhaust line of a semiconductor processing device. The apparatus uses RF energy to form excite the constituents of particulate matter exhausted from a semiconductor processing chamber into a plasma state such that the constituents react to form gaseous products that may be pumped through the vacuum line. The apparatus may include a collection chamber structured and arranged to collect particulate matter flowing through the apparatus and inhibiting egress of the particulate matter from the apparatus. The apparatus may further include an electrostatic collector to enhance particle collection in the collection chamber and to further inhibit egress of the particulate matter.

BACKGROUND OF THE INVENTION

The present invention relates generally to the field of semiconductorprocessing equipment and more specifically to a method and apparatus foreliminating contaminants and residues from inside a vacuum exhaust lineconnected to a processing chamber.

During chemical vapor deposition (CVD) processing, deposition gases arereleased inside a processing chamber to form a thin film layer on thesurface of a substrate being processed. Unwanted deposition on areassuch as the walls of the processing chamber also occurs during such CVDprocesses. Because the residence time in the chamber of individualmolecules in these deposition gases is relatively short, however, only asmall portion of the molecules released into the chamber are consumed inthe deposition process and deposited on either the wafer or chamberwalls.

The unconsumed gas molecules are pumped out of the chamber along withpartially reacted compounds and reaction byproducts through a vacuumline that is commonly referred to as the “foreline.” Many of thecompounds in this exhausted gas are still in highly reactive statesand/or contain residues or particulate matter that can form unwanteddeposits in the foreline. Given time, this deposition build-up ofpowdery residue and/or particulate matter presents a serious problem.First, the build-up poses a safety threat in that the matter is often apyrophoric substance that may ignite when the vacuum seal is broken andthe foreline is exposed to ambient conditions during standard, periodiccleaning operations. Second, if enough of the deposition materialbuilds-up in the foreline, the foreline and/or its associated vacuumpump may clog if it is not appropriately cleaned. Even when periodicallycleaned, matter build-up interferes with normal operation of the vacuumpump and can drastically shorten the useful life of the pump. Also, thesolid matter may backwash from the foreline into the processing chamberand contaminate processing steps adversely effecting wafer yield.

To avoid these problems, the inside surface of the foreline is regularlycleaned to remove the deposited material. This procedure is performedduring a standard chamber clean operation that is employed to removeunwanted deposition material from the chamber walls and similar areas ofthe processing chamber. Common chamber cleaning techniques include theuse of an etching gas, such as fluorine, to remove the depositedmaterial from the chamber walls and other areas. The etching gas isintroduced into the chamber and a plasma is formed so that the etchinggas reacts with and removes the deposited material from the chamberwalls. Such cleaning procedures are commonly performed betweendeposition steps for every wafer or every N wafers.

Removal of deposition material from chamber walls is relatively straightforward in that the plasma is created within the chamber in an areaproximate to the deposited material. Removal of deposition material fromthe foreline is more difficult because the foreline is downstream fromthe processing chamber. In a fixed time period, most points within theprocessing chamber come in contact with more of the etchant fluorineatoms than do points within the foreline. Thus, in a fixed time period,the chamber may be adequately cleaned by the clean process while residueand similar deposits remain in the foreline.

To attempt to adequately clean the foreline, the duration of the cleanoperation must be increased. Increasing the length of the cleanoperation, however, is undesirable because it adversely effects waferthroughput. Also, such residue build-up can be cleaned only to theextent that reactants from clean step are exhausted into the foreline ina state that they may react with the residue in the foreline. In somesystems and applications, the residence time of the exhausted reactantsis not sufficient to reach the end or even middle portions of theforeline. In these systems and applications, residue build-up is evenmore of a concern. Accordingly, there is a need for an apparatus forefficiently and thoroughly cleaning the foreline in a semiconductorprocessing system and a method of doing the same.

One approach that has been employed to clean the foreline relies on ascrubbing system that uses plasma enhanced CVD techniques to extractreactive components in the exhaust gas as film deposits on electrodesurfaces. The scrubbing system is designed to maximize the removal ofreactants as a solid film and uses large surface area spiral electrodes.The spiral electrodes are contained within a removable canister that ispositioned near the end of the foreline between the blower pump andmechanical pump. After a sufficient amount of solid waste has built upon the electrodes, the canisters may be removed for disposal andreplacement.

Problems exist in this prior art method in that the system relies on thelarge surface area of the electrodes to provide an area for depositedsolid matter to collect. To accommodate the large surface area of theelectrodes, the system is necessarily large and bulky. Furthermore,extra expenses are incurred in the operation of this prior art scrubbersystem since the removable canister is a disposable product that must bereplaced and properly disposed. Also, the scrubbing system is locateddownstream from a beginning portion of the vacuum foreline and thus doesnot ensure removal of powdery material or particulate matter thatbuilds-up in this portion of the line.

SUMMARY OF THE INVENTION

The present invention solves the above problems of the prior art byproviding an apparatus that substantially prevents particulate matterand other residual material from building up in an exhaust line. Powderresidue and other particulate matter that would otherwise collects inthe vacuum line during deposition steps is trapped in a collectionchamber and removed through a plasma formed downstream of the reactionchamber. The plasma is formed from reactants in the exhaust residues andexhaust gases pumped through the collection chamber. Constituents fromthe plasma react to form gaseous products that are readily pumpedthrough and out of the exhaust line. The invention also provides amethod for preventing the formation of and ensuring removal of suchdeposition material.

In one embodiment of the apparatus of the present invention, a coilsurrounds a gas passage way defined by a vessel chamber. The coil isconnected to an RF power supply that is used to excite molecules fromparticulate matter and residue within the passageway into a plasmastate. Constituents from the plasma react to form gaseous products thatmay be pumped through the vacuum line.

In another embodiment of the apparatus of the present invention, thepassage way includes a collection chamber between an inlet and outlet ofthe vessel. The collection chamber is structured and arranged to collectparticulate matter flowing through the passage way and inhibit egress ofthe particulate matter from the collection chamber. Particles trapped inthe collection chamber are excited into a plasma state by an RF asdescribed above.

In still another embodiment, the apparatus of the present inventionfurther includes an electrostatic collector positioned within the gaspassage way. The electrostatic collector is designed to collect and trapwithin the passage way electrically charged particulate matter flowingthrough the passage way.

These and other embodiments of the present invention, as well as itsadvantages and features are described in more detail in conjunction withthe text below and attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one embodiment of a simplified chemical vapordeposition apparatus to which the apparatus of the present invention maybe attached;

FIG. 2 illustrates one method of connecting the present invention to thechemical vapor deposition apparatus of FIG. 1;

FIG. 3 illustrates a second method of connecting the present inventionto the chemical vapor deposition apparatus of FIG. 1;

FIG. 4 is a side cross-sectional view of one embodiment of the vacuumline cleaning apparatus of the present invention;

FIG. 5 is a side cross-sectional view of a second embodiment of thevacuum line cleaning apparatus of the present invention;

FIG. 6(a) is a side cross-sectional view of a third embodiment of thevacuum line cleaning apparatus of the present invention;

FIGS. 6(b) and 6(c) are diagrams illustrating the effect of theelectrostatic trap employed in the apparatus of FIG. 6(a) on a particlepumped into the apparatus;

FIG. 7 is a side cross-sectional view of a prototype of the apparatus ofthe present invention used in performing tests evaluating theeffectiveness of the invention;

FIG. 8 is a micrograph showing the amount of residue build-up on asilicon piece inside the vacuum foreline after a 15 second siliconnitride deposition process; and

FIG. 9 is a micrograph showing the size of particulate matter depositedon a silicon piece inside the vacuum foreline during an experimentperformed prior to testing the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS I. ExemplarySemiconductor Processing Chamber

The apparatus of the present invention can be used in conjunction with avariety of different semiconductor processing devices. One suitabledevice, a chemical vapor deposition machine, is shown in FIG. 1 which isa vertical, crosssectional view of a simplified, parallel plate chemicalvapor deposition reactor 10. Reactor 10 includes a gas distributionmanifold 11 for dispersing deposition gases to a wafer, not shown, thatrests on a susceptor 12 in a vacuum chamber 15. Susceptor 12 is highlythermally responsive and is mounted on support fingers 13 so thatsusceptor 12 (and the wafer supported on the upper surface of susceptor12) can be controllably moved between a lower loading/off-loadingposition and an upper processing position 14 which is closely adjacentmanifold 11.

When susceptor 12 and the wafer are in processing position 14, they aresurrounded by a baffle plate having a plurality of spaced holes 23 whichexhaust into an annular vacuum manifold 24. During processing, gas inletto manifold 11 is uniformly distributed radially across the surface ofthe wafer as indicated by arrows 21. The gas is then exhausted via ports23 into the circular vacuum manifold 24 and through a vacuum foreline 31by a vacuum pump system 32. Before reaching manifold 11, deposition andcarrier gases are supplied through gas lines 18 into a mixing chamber 19where they are combined and then sent to manifold 11.

A controlled plasma is formed adjacent to the wafer by RF energy appliedto manifold 11 from RF power supply 25. Gas distribution manifold 11 isan RF electrode, while susceptor 12 is grounded. RF power supply 25 cansupply either single or mixed frequency RF power (or other desiredvariations) to manifold 11 to enhance the decomposition of reactivespecies introduced into chamber 15.

A circular external lamp module 26 provides a collimated annular patternof light 27 through quartz window 28 onto an annular outer peripheralportion of susceptor 12. Such heat distribution compensates for thenatural heat loss pattern of the susceptor and provides rapid thermaland uniform susceptor and wafer heating for effecting deposition.

A motor, not shown, raises and lowers susceptor 12 between a processingposition 14 and a lower, wafer-loading position. The motor, gas supplyvalves (not shown) connected to gas lines 18 and RF power supply 25 arecontrolled by a processor 34 over control lines 36 of which only someare shown. Processor 34 operates under the control of a computer programstored in a memory 38. The computer program dictates the timing, mixtureof gases, chamber pressure, chamber temperature, RF power levels,susceptor position, and other parameters of a particular process.

Typically, any or all of the chamber lining, gas inlet manifoldfaceplate, support fingers 13, and various other reactor hardware isfabricated from material such as anodized aluminum. An example of such aPECVD apparatus is described in U.S. Pat. No. 5,000,113 entitled“Thermal CVD/PECVD Reactor and Use for Thermal Chemical Vapor Depositionof Silicon Dioxide and In-situ Multi-step Planarized Process,” which iscommonly assigned.

The above reactor description is mainly for illustrative purposes, andthe present invention may be used with other CVD equipment such aselectron cyclotron resonance (ECR) plasma CVD devices, induction coupledRF high density plasma CVD devices, or the like. The present inventionmay also be used with thermal CVD devices, plasma etching devices andphysical vapor deposition devices. The apparatus of the presentinvention and the method for preventing deposition build-up within avacuum line is not limited to any specific semiconductor processingapparatus or to any specific deposition or etching process or method.

II. Exemplary Semiconductor Processing Operations

During semiconductor processing operations such as chemical vapordeposition processes carried out by CVD reactor 10, a variety of gaseouswaste products and contaminants are exhausted from vacuum chamber 15into vacuum line 31. Depending on the particular operation beingperformed, these exhaust products may include particulate matter such aspartially reacted products and byproducts that leaves a residue orsimilar powdery material within the foreline as it is exhausted throughthe foreline. For example, during the deposition of a silicon nitridefilm using silane (SiH₄), nitrogen (N₂) and ammonia (NH₃) as precursors,residue in the form of a brown powder composed of Si_(x)N_(y)H_(z),Si_(x)H_(y), SiO_(x) and elemental silicon has been observed in theforeline. It is believed that this residue build-up is from half-reactedbyproducts of the reaction of SiH₄+N₂+NH₃. Similar residues are alsoformed during the deposition of silicon nitride layers using otherprecursor gases or liquids such as disilane (S₂H₆) or organic sources.Residue build-up may also occur during the deposition of oxynitridefilms and silicon oxide films among other layers and may also occurduring plasma etching and other process steps.

The present invention prevents build-up of such residues and particulatematter by trapping the particulate matter in a collection chamber andexciting reactant gases exhausted through the vacuum foreline and theresidual and particulate matter within the line into a plasma state. Theplasma reacts with and etches residues and particulate matter that tendsto build-up in the foreline to form gaseous products and byproducts thatmay be pumped through the vacuum line without forming deposits orcondensing within the line.

III. Exemplary Embodiments of the Present Invention

As shown in FIG. 2, which is a vertical, cross-sectional view of thesimplified CVD apparatus of FIG. 1 fitted with the apparatus of thepresent invention, the apparatus of the present invention is positioneddownstream from the exhaust gas source—the processing chamber. Theapparatus may either connect to or replace a portion of the vacuumforeline.

In FIG. 2, a downstream plasma cleaning apparatus 40 (hereinafterreferred to as “DPA 40” or “the DPA”) is fitted between vacuum pumpsystem 32 and vacuum manifold 24 along a portion of vacuum line 31.Because of its position, gases exhausted from vacuum chamber 15necessarily passes through DPA 40. DPA 40 may be positioned at anylocation along vacuum line 31, but preferably, DPA 40 is positioned asclose as possible to exhaust manifold 24 so that gases exhausted fromchamber 15 pass through DPA 40 before passing through any portion ofvacuum line 31.

In operation, as deposition gases are exhausted from vacuum chamberthrough vacuum line 31, particulate matter and residue from the gasesare deposited on the interior surface of the vacuum line. Removal of theparticulate matter and residues may be achieved by activating DPA 40.For such removal, DPA 40 may be turned ON during both deposition andclean operations or may be activated only during the clean procedure.

When activated, DPA 40 creates a voltage field that excites molecules ofresidual matter deposited on the interior surfaces of the DPA andmolecules of exhaust gases passing through the DPA into a plasma state.The plasma enhances decomposition of the matter within DPA 40 intogaseous products and byproducts that may be pumped out through theforeline thus preventing particle deposition or residue build-up. Forexample, in the case where residue build-up within DPA 40 is in the formof the brown powder comprising Si_(x)N_(y)H_(z), Si_(x)H_(y), SiO_(x)and elemental silicon as described above in respect to silicon nitridedeposition, it is believed that the plasma formed by DPA 40 breaks theresidue down into gaseous components such as SiF₄, CO and CO₂, NO andN₂O, and O₂.

In addition to collecting residue by normal deposition within DPA 40,various embodiments of DPA 40 are specifically designed to trapparticulate matter exhausted from chamber 15 within the DPA so that thematter cannot be deposited outside the DPA. Trapping is done usingmechanical and/or electrostatic trapping mechanisms as described in moredetail below. Once trapped, particulate matter remains in DPA 40 untilit reacts with active species in the plasma to form gaseous byproductsthat are then pumped through vacuum line 31.

The voltage field created within DPA 40 to form the plasma can begenerated using a variety of known methods such as capacitively coupledelectrodes, inductively coupled coils or ECR techniques. Because of itscompact size and capacity to create relatively high voltage fields, itis preferable, however, to create the voltage field with an inductivecoil such as a helical resonator coil. Such coils are well known tothose of ordinary skill in the art and may be designed according tocriteria set forth in any of a number of well known textbooks such asMichael A. Lieberman and Allan J. Lichtenberg, “Principles of PlasmaDischarges and Materials Processing,” pp. 404-410 John Wiley & Sons(1994), which is hereby incorporated by reference.

The helical resonator coil can be made out of a high conductivity typemetal such as copper, nickel, or gold or similar conducting material. Toproperly resonate the coil, it is important that the length of the coilbe about or slightly longer than ¼ of the wavelength of the applied RFsignal. A coil of this length creates a stronger and more intensevoltage field that further enhances decomposition.

The helical resonator coil is connected at one end to an RF power supplyand at the opposing end to a ground potential. To ensure completereaction of material passing through and/or deposited within DPA 40, theDPA must be driven by the RF power supply at a level sufficient to forma plasma. Generally, a power level of between 50-1000 Watts or more canbe employed, and preferably a power level of between 100-400 Watts isused. The actual power level selected should be determined by balancinga desire to use a high power level to form an intense plasma and adesire to use a low power level to save energy costs and allow use ofsmaller, less expensive power supplies. Plasma uniformity and othercharacteristics important in conventional plasma enhanced CVD reactorsare of secondary concern in the formation of the DPA plasma.

The power supply driving DPA 40 is operated at a frequency range fromabout 50 KHz to about 200 MHz or more and is usually operated in therange of about 50 KHz to 60 MHz. RF power supply can be supplied fromeither a single frequency RF source or a mixed frequency RF source. Thepower output of the supply will depend on the application for which theDPA is used and on the volume of the gas to be treated in DPA 40. RFpower can be derived from RF power supply 25 or can be supplied by aseparate RF power supply that drives only DPA 40. Additionally, assumingmultiple processing chambers are present in a clean room, the multipleDPAs connected to the chambers may all be driven by a separate,dedicated DPA RF power supply that is connected to an appropriate numberof RF power splitters.

The length and size of DPA 40 can vary. In some applications, DPA 40 canbe only 4-6 inches long or even shorter, while in other applications,DPA 40 can be the entire length of vacuum line 31 (4-5 feet or longer)thus replacing the line. A longer DPA will collect and thus be able toremove more particulate matter than a shorter, identically designed DPA.DPA design must balance space considerations with residue collectingefficiency. Shorter DPAs that include an advanced trapping mechanism,however, are able to collect and trap 99.9% of all particulate matterexhausted from the processing chamber making length a less importantfactor. Because the length of the coil should be slightly longer than ¼of the RF wavelength, there is a direct relationship between the coillength and RF frequency used. Longer coils require lower frequency RFpower signals.

While it was previously described that DPA 40 may be turned ON and OFFduring specific periods of a processing procedure, the DPA may also beconfigured as a passive device. As a passive device, DPA 40 is suppliedcontinuously with a sufficient RF power signal so that no specialcontrol signals or processor time need be devoted to turning the DPA ONand OFF.

As previously mentioned, if configured as an active device, power issupplied to DPA 40 during the time at which a chamber clean operationtakes place. Optionally, RF power may also be supplied during the periodin which film deposition occurs in chamber 15. Control of the timingaspects of DPA 40 when configured as an active device is generallyperformed by processor 34 through the application of control signalssent over control lines 36.

As shown in FIG. 3, it is possible to connect two or more DPAs to vacuumline 31. Such a configuration might be used, for example, to furtherprotect vacuum pump 32 from residue build-up. In the configuration shownin FIG. 3, a second DPA 41 is positioned downstream from DPA 40 justbefore pump 32. If any particulate matter escapes DPA 40, the matter canbe trapped and converted into gaseous form within DPA 42. DPA 40 and 42can both be driven by a single RF power supply 44 with the power bebeing split by a splitter 46. Optionally, DPA 40 and 42 may each bedriven by separate RF power supplies or may both be driven from the mainRF power supply connected to processing chamber 10.

A number of different embodiments of the apparatus of the presentinvention may be constructed. Three such embodiments are illustratedbelow for exemplary purposes. In no way should it be construed that thepresent invention is limited to these specific embodiments.

1. Single Tube Embodiment

FIG. 4 is a cross-sectional view of an embodiment of DPA 40. In FIG. 4,DPA 40 includes a tube 50 through which exhaust gases from processingchamber 15 flow as they pass through DPA 40. Tube 50 is a cylindricaltube made out of an insulating material such as ceramic, glass orquartz. In a preferred embodiment, tube 50 is made out of a ceramicmaterial that is does not react with etchant gases, such as fluorine,used in the clean steps. Also, tube 50 has approximately the sameinterior diameter as the interior diameter of vacuum line 31. In otherembodiments, tube 50 need not necessarily be in cylindrical form and mayinstead have angular, planar or elliptical or similarly curved interiorsurfaces. In these and other embodiments, the interior diameter of tube50 may also be either larger or smaller than the interior diameter ofvacuum line 31.

A coil 52 is wound around the exterior of tube 50 and connected to an RFpower supply at point 56 and connected to a ground potential at point57. Exhaust material passing through tube 50 and exhaust materialdeposited within the tube is excited into a plasma state by theapplication of a voltage from the RF power supply to coil 52. In theplasma state, constituents from the exhaust material react to formgaseous products that may be pumped out of DPA 40 and vacuum line 31 bypump system 32 as described above. Coil 52 is a standard helicalresonator coil as previously discussed and may be wound within theinterior of tube 50 rather than external to the tube.

An outer container 54 surrounds tube 50. Container 54 serves at leasttwo purposes. First, it shields CVD processing apparatus 10 and otherequipment from the voltage and noise signals generated by coil 52.Second, if ceramic tube 50 were to break or crack or if the vacuum sealin tube 50 is broken in another manner, container 54 provides a secondseal preventing the exhaust gases from escaping. Container 54 can bemade out of a variety of metals such as aluminum or steel or othercompounds and is preferably grounded for shielding effect. Upper andlower flanges 57 and 58, respectively, connect DPA 40 to vacuum manifold24 and vacuum line 31 while maintaining a vacuum seal.

Standard RF power supplies are designed to work off an impedance of 50ohms as a load. Accordingly, the point of contact for the RF powersupply to coil 52 (point 56) should be selected so that coil 52 has animpedance of 50 ohms. Other course, if the power supply required anotherimpedance level, point 56 should be chosen accordingly.

Coil 52 is driven by the RF power supply at a power level of 50 Watts orgreater. Under such conditions, plasma generation is at a maximum anduniformity is not a concern. The actual voltage generated by coil 52depends on a number of factors such as the power used by the RF powersupply, length and winding spacing coil 52 and the resistance of thecoil among other factors. Since voltage is spread evenly along the coil,determining the voltage level for the entire coil can be done bydetermining the level between the points at which the coil is connectedto ground and the RF power supply (points 55 and 56). For example, if aparticular coil is four times as long as the portion of the coil betweenpoints 55 and 56, the total voltage of the coil will be four times thevoltage level between points 55 and 56.

The coil, power level and applied RF frequency should be selected sothat a strong, intense plasma is formed within tube 50, but also so thatthe voltage generated by coil 52 does not exceed a level at whichcurrent will arc from the coil to container 54. It is possible to put aninsulating material between container 54 and coil 52 if arcing is aproblem for a particular DPA. For simplicity of design, however, it ispreferable to have the space between container 54 and coil 52 filledwith air.

2. A First Mechanical and Electrostatic Trap Embodiment

FIG. 5 is a cross-sectional view of a second embodiment of DPA 40. Theembodiment of DPA 40 shown in FIG. 5 includes a first inner ceramic tube60 and a second outer ceramic tube 62. The end of tube 60 is within thecylindrical space of tube 62 so that gas flow through DPA 40 is as shownin arrows 64.

A helical resonator coil 66 is wrapped around the exterior of tube 62and connected to an RF power supply 68 as described in relation to theembodiment of FIG. 4. Coil 66 could also be wound within the interior oftube 62 or around the exterior or interior of tube 60.

A shell 68, similar to container 50 above, encloses both inner and outertubes 60 and 62. Outer tube 62 may be supported by connections to eitherinner tube 60 or shell 68. In either case, it is important that asupport structure for outer tube 62 allow the effluent gas stream topass through DPA 40. To this end, the support structure may be a planeof ceramic material between tubes 60 and 62 having a plurality ofperforated holes, may consist or only three of four slender connectionsor fingers extending between tubes 60 and 62, or may be designed innumerous other equivalent manners. A structure including perforatedholes can help collect and trap particulate matter within a collectionarea 70 described below. The structure should be designed, however, sothat the holes are large enough so as to not reduce the flow rate ofgases pumped through DPA 40.

The design of this embodiment of DPA 40 enhances the trapping andtherefore decomposition of particulate matter. The design includescollection area 70 of tube 62 that acts as a mechanical trap collectingand holding particles in the exhaust gas stream so that they cannot passthrough the remainder of the DPA into vacuum line 31. The particles areheld in the trap and subjected to the plasma until they disassociate orbreak down under the formed plasma.

The operation of the trap portion of this embodiment of DPA 40 relies inpart on gravitational forces that act to hold the particulate matterwithin the trap despite an effluent gas flow path that attempts to sweepthe particles through the DPA device into the vacuum line. Thus, inpart, the effectiveness of DPA 40 depends on the ability of exteriortube 62 to prevent particles from leaving tube 62 until they are reactedinto gaseous products. To this end, it is important that DPA 40 bepositioned so that collection area 70 is downward from the inlet to theDPA and that the length of exterior tube 62 be sufficient to create thistrap in combination with gravitational forces.

Increasing the cross-sectional area of the gas passage ways along aplane 76 within DPA 40 further helps trap particulate matter. The rateof flow for an effluent gas stream in any given deposition process isgenerally constant. Thus, increasing the cross-sectional area of one ormore of the passage ways decreases the velocity of particles in the gasstream which correspondingly reduces the neutral drag force on theparticles. A given particle is trapped by gravitational forces withinthe gravity trap of DPA 40, if the gravitational force on the particleexceeds the neutral drag force.

To further enhance the effectiveness of the mechanical trap, anelectrostatic collector 72 can be positioned near collection area 70 toattract exhausted particulate matter which is electrically charged.Electrostatic collector 72 may be a small electrode connected to a DC orAC power supply of between 100-3000 volts. Of course the polarity andamount of charge applied to electrostatic collector 72 is applicationspecific and depends on the polarity type and typical charge level ofexhausted particulate material in an individual application.

A variety of different electrostatic trapping devices may be employed inthe present invention. Details of a second, preferred embodiment of suchan electrostatic collector and discussed in detail below with respect toFIGS. 6(a) and 6(b).

3. A Second Mechanical and Electrostatic Trap Embodiment

FIG. 6(a) is a cross-sectional view of a third embodiment of DPA 40. Theembodiment of FIG. 6(a) employs a mechanical trap design similar to theembodiment of FIG. 5 and also employs a modified electrostatic trap.Also, effluent gas is exhausted through a side flange 80 locatedadjacent to upper flange 81 rather than opposite the upper flange.Flange 80 is positioned to create a vacuum seal with outer casing 84rather than exterior tube 86. Casing 84 is made from a metal or similarmaterial while tube 86 is made out of an insulating material such asceramic.

RF power is supplied to the DPA of this embodiment through an outer coil87 that is designed to have an impedance of 50 ohms between the point ofconnection 88 to the RF supply and point 89 (ground). As above, coil 87should be designed to have an impedance of 50 ohms so that the coil maybe driven by a standard RF power supply. An inner coil 90 is woundwithin an inner tube 85. Inner coil 90 receives by induction the RFsignal supplied on outer coil 87 and creates the voltage field necessaryto drive the plasma reaction.

A central wire 92 runs through the center of inner tube 85 and a voltagepotential is created between central wire 92 and inner coil 90 toelectrostatically trap particulate matter passing through the DPA. Thevoltage potential can be created using numerous different approaches. Ineach approach, center wire 92 and coil 90 act as electrodes. In oneembodiment, center wire 92 is grounded and a positive DC or AC voltageis applied to coil 90. As shown in FIG. 6(b), in the case where exhaustparticles 94 are negatively charged, the particles are attracted by thevoltage field (F_(elec)) created by wire 92 and coil 90 and collect atpositions 95 on the positively charged coil. A similar result can beachieved if coil 90 is grounded and a negative voltage is applied tocenter wire 92. In this case, however, wire 92 repels negatively chargedparticles toward coil 90.

In another embodiment, a positive DC or AC voltage is applied to centerwire 92 and coil 90 is connected to a ground potential. In thisapproach, the negatively charged particles are collected at positions 96on positively charged wire 92 as shown in FIG. 6(c). A similar resultcan be achieved if a negative voltage is applied to coil 90 and centerwire 92 is grounded. In this case, coil 90 repels the negatively chargedparticles toward wire 92.

In still other embodiments, neither wire 92 or coil 90 are grounded andinstead both are connected to voltage sources that create a positive ornegative voltage from wire 92 relative to coil 90. Of course, in thecase where positively charged particulate matter is present, this mattermay be collected on the electrode opposite the electrode the negativelycharged matter is collected on.

Also, particles may be collected by electrostatic forces in cases wherethe particulate matter includes both positively and negatively chargedparticles. In such a case, it is preferable to apply an AC voltage toone electrode and ground the other. For example, when an AC voltage isconnected to center wire 92 and coil 90 is grounded, positiveparticulate matter is repelled from the wire toward coil 90 during thepositive half-cycle. During the negative half-cycle, however, negativeparticulate matter is repelled from the wire and collected on coil 90.

In any of the above cases, the electric field can be a voltage betweenthe two electrodes of between 100 and 5000 volts. Preferably, thevoltage between the electrodes is between 500 volts (DC) to 5000 volts(AC). Whether particles are attracted away from central wire 92 tocollect on coil 90 or vice versa depends on the polarity of theparticles and the charges applied to coil 90 and wire 92.

Because this design relies on the voltage potential created between coil90 and center wire 92, coil 90 should be positioned inside inner tube 85to obtain maximum particle collection so as to not be separated fromwire 92 by the insulating material of the tube. Being situated insidetube 85, coil 90 and center wire 92 come in contact with a variety ofhighly reactive species such as fluorine. Accordingly, it is importantthat coil 90 and wire 92 be made of a suitable conductive material, suchas nickel, that does not react with such species. It is important tonote that coil 90 carries both a voltage potential to attract or repelparticles and RF power signal in this embodiment.

The electrostatic collector and mechanical trap combination provides aparticularly effective mechanism to prevent deposition build up invacuum line 31. The mechanical trap section is particularly effective intrapping relatively large particles present in the effluent gas streambecause these particles are more likely to be held within exterior tube62 by gravitational forces. The electrostatic trap, on the other hand,is particularly effective at collecting and trapping smaller particlesin the effluent gas stream that may otherwise not be collected by justthe mechanical trap.

As an example, in the deposition of silicon nitride as described above,particles ranging in size from 1 μm in diameter to 1 mm in diameter ormore have been observed. When these particles are in the exhaust line,two forces of importance act on the particles: a gravitationalattraction force (F_(g)) and a neutral drag force (F_(nd)) resultingfrom the gas motion. For large particulate matter, such as particleslarger than 100 μm in diameter, the major interaction is thegravitational force, so the mechanical trap is particularly effective.For smaller particles, however, the drag force of the gas can be higherthan the gravitational force. Consequently, the electric field developedbetween the two electrodes of the electrostatic trap applies asupplementary force (F_(elec)), perpendicular to the trajectory of theparticulate. This force can be two or more orders of magnitude largerthan both the gravitational and drag forces for very small particulates,such as those less than 10 μm in diameter, resulting in a very highcollection efficiency.

IV. Experimental Use and Test Results

To demonstrate the effectiveness of the present invention, experimentswere performed in which a prototype DPA 40 was attached to a Precision5000 chamber outfitted for 8 inch wafers. The Precision 5000 chamber ismanufactured by Applied Materials, the assignee of the presentinvention.

In the experiments, the prototype DPA was similar to DPA 40 shown inFIG. 3 except for the design of the lower flange used to connect the DPAto the foreline. A cross-sectional view of the prototype DPA and lowerflange is shown in FIG. 7. As shown in FIG. 7, a lower flange 100redirected the exhaust gases flowing through the DPA into the forelineat an angle of approximately 90 degrees. The flange also was fitted witha quartz window opposite the foreline connection so that depositionmaterial that built-up on a bottom portion 104 of flange could beobserved. This design of the lower flange in the prototype DPA had theadded benefit of trapping particulate matter in area 104 in a mannersimilar to but not as effective as the mechanical bucket trap designs inthe embodiments of DPA 40 shown in FIGS. 5 and 6.

The prototype device included a quartz tube 106 having a coil 108 madeout of ⅜ inch copper tubing wrapped around the exterior of the quartztube. The total length of coil 108 was approximately 25 feet and a 13.56MHz power supply was driven at various power levels as explained in thedescription of the experiments below. Quartz tube 106 and coil 108 weresealed within an aluminum container 110. The entire length of theassembly was approximately 14 inches, and the width of the assembly wasapproximately 4.5 inches.

Before experiments were performed testing the effectiveness of the DPA,experiments were performed to determine the composition of residuedeposited in the processing chamber by a silicon nitride deposition stepfollowed by a fluorine clean step. The composition of the residue wasdetermined for two different silicon nitride deposition/fluorine cleanoperations process sequences. In each process sequence, the siliconnitride deposition step was identical while the clean step was based ona CF₄ chemistry in the first sequence and on an NF₃ chemistry in thesecond sequence.

The silicon nitride film was deposited on a wafer by subjecting thewafer to a plasma of silane (SiH₄), nitrogen (N₂) and ammonia (NH₃)gases. SiH₄ was introduced into the chamber at a flow rate of 275 sccm,N₂ was introduced into the chamber at a rate of 3700 sccm and NH₃ wasintroduced at a rate of 100 sccm. The plasma was formed at a pressure of4.5 torr, at a temperature of 400° C., using a 13.56 MHz RF power supplydriven at 720 Watts. The silicon nitride deposition process depositedlasted approximately 75 seconds which was sufficient to deposit a filmof approximately 10,000 Å on the wafer.

For the first measurement, after the silicon nitride deposition step wascompleted and the wafer removed from the chamber, the chamber wascleaned with a CF₄ and N₂O plasma for 120 seconds. The ratio of CF₄ toN₂O was 3:1 with the CF₄ being introduced at a rate of 1500 sccm and N₂Obeing introduced at a rate of 500 sccm. During the clean step, thechamber was maintained at a temperature of 400° C. and at a pressure of5 torr. The plasma was formed with a 13.56 MHz power supply powered at1000 Watts.

For the second measurement, the chamber was cleaned with a plasma formedfrom NF₃ and N₂O and N₂ precursor gases. The ratio of NF₃ to N₂O wasapproximately 5:2:10 with NF₃ being introduced at a rate of 500 sccm,N₂O being introduced at a rate of 200 sccm, and N₂ being introduced at arate of 1000 sccm. The chamber was maintained at a temperature of 400°C. and a pressure of 5 torr during the clean step, which lasted forapproximately 95 seconds. Plasma formation was achieved with a 13.56 MHzpower supply powered at 1000 Watts.

As evident in Table 1 below, the residue build-up from the siliconnitride deposition/CF₄ chamber clean process sequence was a brownishpowder while the residue build-up from the silicon nitridedeposition/NF₃ chamber clean sequence was a yellow-white powder.

TABLE 1 Residue Formation From Silicon Nitride Deposition/Fluorine CleanProcesses Residue Composition Cleaning Residue C O N Si F H Process Typeat % at % at % at % at % at % CF₄ + N₂O brown 0.2 6.8 13 42 1 37 powderNF₃ + N₂O + yellow-white 0 1 12 8.5 38.5 40 N₂ powder

After the composition of residue build-up in the chamber was determined,an experiment was performed to determine the grain size of the residualpowder. For this experiment, a silicon piece was placed within theforeline to collect material deposited there from the depositionprocess. It was observed that even after a 15 second deposition process,a residue build-up in the form of a brown powder normally occurs invacuum line 3. A micrograph showing this residue build-up is shown asFIG. 8. The brown powder was made up of Si_(x)N_(y)H_(z), Si_(x)H_(y),SiO_(x) and elemental silicon residues. As shown in FIG. 9, a micrographof a typical residue particle, the elementary grain size of the powderwas approximately 1-50 μm in diameter. Further experiments showed thatthe grain size of the powder increased with deposition time to formaggregates as large as 1 mm in diameter for a 90 second deposition step.

The effectiveness of the DPA was tested in three separate experiments.In each experiment 100 wafers were processed in a silicon nitridedeposition/CF₄ fluorine clean operation sequence performed in aPrecision 5000 chamber having the prototype DPA connected between thevacuum exhaust manifold and the foreline. The prototype the DPA was keptOFF during the deposition sequence of each experiment and was turned ONand powered by a 13.56 RF power supply during the fluorine cleansequence. When OFF during deposition, particles collected along theinterior of tube 106 as shown in FIG. 7 as areas 112. These particleswere then removed from tube 106 when the DPA was activated during theclean sequence. The conditions for each of the three experiments aresummarized in Table 2 below.

TABLE 2 Foreline Cleaning Results Experiment 1 Experiment 2 Experiment 3RF Frequency 13.56 MHz 13.56 MHz 13.56 MHz RF Power 200 500 500 CF₄ Flow1500  2000  2500  N₂O Flow 500 500 500 Result Residue #1 in Residue #2in Residue Exterminated Table 3 Table 3

In the first experiment, the fluorine clean sequence was 135 seconds andthe DPA was driven at 200 Watts. CF₄ was introduced into the processingchamber at a rate of 1500 sccm and mixed with N₂O introduced into thechamber at a rate of 500 sccm (a 3:1 ratio). After 100 deposition/cleansequences, the DPA was examined and found to be free of all residue anddeposits. In the angular flange at the bottom of the DPA, a small amountof a residue build-up had collected. The atomic concentration of thisresidue build-up was measured and is summarized in Table 3 below. Themajority of silicon in the residue was contained in the form of siliconoxide and approximately half the nitrogen was contained in a siliconnitride film with the other half being in the form of ammonia.

In the second experiment, the fluorine clean sequence was shortened to120 seconds and voltage at which the DPA was driven was increased to 500Watts. CF₄ was introduced into the processing chamber at a rate of 2000sccm and mixed with N₂O introduced into the chamber at a rate of 500sccm (a 4:1 ratio). After 100 deposition/clean sequences, the DPA wasexamined and found to be free of all residue and deposits. A smallamount of a residue build-up had collected in the angular flange. From avisual inspection, the amount of residue build-up was approximately 80%less than the amount of build-up in the first experiment, however.

The atomic concentration of this residue build-up was measured and isalso summarized in Table 3 below. As evident from the table, the residuefrom this experiment contained a significantly higher concentration offluorine than the residue from the first experiment. The fluorineconcentrated residue provides more fluorine species for the plasma andthus makes the residue easier to clean during further DPA activation. Itshould also be noted that the overwhelming majority of silicon in theresidue from this experiment was contained in the form of silicon oxideand the overwhelming majority of nitrogen was present in the form ofammonia.

The third experiment proved that the residue can be completelyeliminated from both the DPA and the angular flange where residue tendedto collect during the first and second experiments. In this thirdexperiment, the fluorine clean sequence was 120 seconds long and voltageat which the DPA was driven was increased to 500 Watts. The rate atwhich CF₄ was introduced into the processing chamber was increased to2500 sccm and mixed with N₂O introduced into the chamber at a rate of500 sccm (a 5:1 ratio). After 100 deposition/clean sequences, the DPAand angular flange were examined and both were found to be free of allresidue and deposits.

The results of these experiments in the way of residue presence andcomposition is summarized in Table 3 below.

TABLE 3 Residue Collected at Bottom of DPA Atomic concentration % Si %present as N % present as C O N Si F H elem. nit. ox. nitride NH₃Residue #1 3.4 44.8  7.4 31.4 13.1 N/A 13.9 20 66.1 48.6 51.4 Residue #24.8 20.5 15.2 19.8 39.8 N/A  4.2 3.3 92.5  3.7 96.3 Residue #3 NONE NONENONE NONE NONE NONE NONE NONE NONE NONE NONE

Having fully described several embodiments of the present invention,many other equivalent or alternative devices for and methods of removingparticulate matter from a vacuum line according to the present inventionwill be apparent to those skilled in the art. Additionally, although thepresent invention has been described in some detail by way ofillustration and example for purposes of clarity and understanding, itwill be obvious that certain changes and modifications may be practiced.For example, while the mechanical particle trap of the present inventionwas described with respect an inner passage way surrounded by an outerpassage way, the trap could be created with a first passage wayperimetrically contained within a second passage way, but insteadextending away from or upward the first passage way. These equivalentsand alternatives along with the understood obvious changes andmodifications are intended to be included within the scope of thepresent invention.

What is claimed is:
 1. A semiconductor processing apparatus comprising:a housing for forming a processing chamber, said housing having anexhaust port for exhausting gases from said processing chamber; asubstrate support adapted to support a substrate within said processingchamber; a gas distribution system configured to introduce gases intosaid processing chamber; a heater configured to heat said substrate; avacuum system configured to control the pressure within said processingchamber; an exhaust line, coupled to said exhaust port and to saidvacuum system, through which gases are exhausted from said processingchamber; and a particle collector, coupled to said exhaust line, fortrapping and removing particulate matter present in gases exhausted fromsaid processing chamber to prevent build-up of deposition materialwithin said exhaust line, said particle collector comprising a vesselchamber defining a gas passageway having an inlet and an outlet and acollection area between the inlet and the outlet, the collection areabeing structured and arranged to collect particulate matter flowingthrough the gas passageway and to inhibit egress of the particulatematter from the particle collector; and a plasma formation system,operatively coupled to form a plasma within at least a portion of saidgas passageway in order to remove particulate matter deposited ortrapped within said gas passageway.
 2. The apparatus of claim 1 whereinsaid plasma formation system comprises a coil, surrounding at least aportion of said gas passageway, and an RF power supply, operativelycoupled to said coil, for supplying RF power to said coil.
 3. Theapparatus of claim 1 wherein vessel chamber defines a flow passagecommunicating the collection area with the outlet, the flow passageextending at least partially upwards from the collection chamber toinhibit egress of the particulate matter from the chamber.
 4. Theapparatus of claim 3 wherein the collection area is defined by a first,lower wall in communication with the inlet of the gas passageway and asecond, perimetrical wall contiguous with and extending upwardly fromthe first wall.
 5. The apparatus of claim 4 wherein the vessel chambercomprises a shaft defining an inner lumen in communication with the gaspassageway inlet and having a lower opening communicating with andvertically spaced above the first wall of the collection area fordischarging particulate matter into the area, the perimetrical wallcircumscribing the shaft and defining an annular, vertical flow passagetherebetween in communication with the outlet of the gas passageway. 6.The apparatus of claim 5 wherein the perimetrical wall defines an outletport in communication with the annular flow passage for discharging thegaseous products from the vessel chamber, the outlet port beingvertically spaced above the collection area.
 7. The apparatus of claim 5further comprising an outer housing surrounding the shaft and thecollection area, the outer housing defining a receiving chamber incommunication with the annular, vertical flow passage and an outlet portin communication with the receiving chamber for discharging the gaseousproducts from the vessel chamber, the outlet port being verticallyspaced below the collection area.
 8. The apparatus of claim 1 furthercomprising an electrostatic collector, coupled to said vessel chamber,to collect electrically charged particulate matter that may be presentin gases passing through said vessel chamber.
 9. The apparatus of claim8 wherein said electrostatic collector is positioned with saidcollection area.
 10. The apparatus of claim 1 wherein said vesselchamber comprises a first passageway defining a path of flow in a firstdirection for gases passing through said vessel chamber and a secondpassageway defining a flow path in a second direction different fromsaid first direction, said first and second passageways being arrangedsuch that gases flow into said vessel chamber through said inlet port,through said first passageway, through said second passageway and thenexit said vessel chamber through said outlet port.
 11. The apparatus ofclaim 10 wherein said first passageway and second passageway aredesigned to create said collection area to trap particulate matter thatmay be present in gasses passing through said vessel chamber.
 12. Theapparatus of claim 11 wherein said first direction is substantiallyopposite said second direction.
 13. A method of minimizing depositionwithin an exhaust line connected to a semiconductor processing chamber,said method comprising; passing gases exhausted from said processingchamber through a vessel chamber defining a gas passageway that isfluidly coupled to said exhaust line; trapping particulate matterpresent in said exhausted gases within said gas passageway with anelectrostatic collector; and striking a plasma in said gas passageway toremove particulate matter deposited or trapped within said vesselchamber.
 14. The method of claim 13 wherein said step of trapping saidparticulate matter is performed in conjunction with a substrateprocessing operation performed in the substrate processing chamber andwherein said step of striking said plasma is performed in conjunctionwith a clean operation of the substrate processing chamber.
 15. Themethod of claim 14 wherein said substrate processing operation comprisesa step of depositing a layer of material by chemical vapor deposition.16. In a processing chamber in which silicon nitride is deposited bychemical vapor deposition onto a substrate, a method of minimizingresidue build-up from said silicon nitride deposition in an exhaust linecoupled to said processing chamber, said method comprising the steps of:pumping gases exhausted from said processing chamber through a vesseldefining a fluid conduit having an inlet and an outlet; collectingparticulate matter present in said exhausted gases in a collection areabetween the inlet and outlet of said vessel; forming a plasma in saidcollection area to react said collected particulate matter into gaseousproducts; and pumping said gaseous products from said vessel.
 17. Themethod of claim 16 wherein said particulate matter comprises partiallyreacted silicon containing products and byproducts from said siliconnitride deposition.
 18. The method of claim 17 wherein partially reactedsilicon containing products and byproducts comprise Si_(x)N_(y)H_(z),Si_(x)H_(y), SiO_(x) and elemental silicon.
 19. The method of claim 16wherein said plasma is formed during a chamber clean operation in whicha reactant gas is introduced into said processing chamber and pumpedthrough said vessel to etch material deposited from said silicon nitridedeposition step.
 20. The method of claim 19 wherein said plasmaformation is selectively turned ON and OFF during operation of saidprocessing chamber.
 21. The method of claim 20 wherein said plasmaformation is turned OFF during deposition of said silicon nitride filmand turned ON during said chamber clean operation.
 22. The method ofclaim 21 wherein said ON and OFF sequence is performed sequentially fordeposition of silicon nitride layers on a plurality of wafers.
 23. Themethod of claim 16 wherein said collecting step uses gravitation forcesto collect particles in said collection area.
 24. The method of claim 16wherein said collecting step uses electrostatic forces to collectparticles in said collection area.
 25. The method of claim 16 whereinsaid collecting step uses gravitational and electrostatic forces tocollect particles in said collection area.
 26. The method of claim 16wherein said step of forming a plasma forms said plasma by applying RFpower to a coil surrounding said collection area.
 27. The method ofclaim 26 wherein said RF power is continuously supplied to said coilduring sequential deposition and clean steps for a plurality of wafers.28. In a substrate processing chamber in which a layer is deposited bychemical vapor deposition onto a substrate, a method of minimizingbuild-up of residue from said chemical vapor deposition step in anexhaust line coupled to said processing chamber, said method comprising:pumping gases exhausted from said processing chamber during saidchemical vapor deposition step through a vessel defining a fluid conduithaving an inlet and an outlet; collecting particulate matter present insaid exhausted gases in a collection area between the inlet and outletof said vessel during said chemical vapor deposition step; forming aplasma in said collection area during a clean operation in saidsubstrate processing chamber to convert said particulate mattercollected in said collection area into gaseous products; and pumpingsaid gaseous products from said vessel.
 29. A method of minimizingresidue build-up within an exhaust line connected to a substrateprocessing chamber, said method comprising: exhausting gases from saidsubstrate processing chamber through a collection chamber fluidlycoupled to an outlet of said substrate processing chamber; collectingparticulate matter from said exhausted gases in said collection chamber;forming a plasma in said collection chamber to react said collectedparticulate matter into gaseous products; and pumping said gaseousproducts from said collection chamber.
 30. The method of claim 29wherein said particulate matter comprises partially reacted siliconcontaining products and byproducts.
 31. The method of claim 29 whereinsaid plasma is formed during a chamber clean operation in which anetchant gas is introduced into said processing chamber and pumpedthrough said collection chamber.
 32. The method of claim 29 wherein saidplasma is selectively ignited and extinguished during operation of saidprocessing chamber.
 33. The method of claim 29 wherein said plasma insaid collection chamber is formed during a chamber clean operation andnot formed during a film deposition step.
 34. The method of claim 29wherein said particulate matter is collected in said collection chamberusing gravitation forces.
 35. The method of claim 29 wherein saidparticulate matter is collected in said collection chamber usingelectrostatic forces.
 36. The method of claim 29 wherein saidparticulate matter is collected in said collection chamber using acombination of gravitational and electrostatic forces.
 37. The method ofclaim 29 wherein said particulate matter is collected in said collectionchamber in conjunction with a substrate processing operation performedin the substrate processing chamber and wherein said plasma is formed insaid collection chamber in conjunction with a clean operation of thesubstrate processing chamber.
 38. The method of claim 37 wherein saidsubstrate processing operation comprises a depositing a layer ofmaterial by chemical vapor deposition over a substrate disposed in saidsubstrate processing chamber.
 39. The method of claim 29 wherein saidcollection chamber includes a fluid conduit having a plurality of bends.40. The method of claim 39 wherein a portion of said fluid conduit formsa serpentine path.